Hello
Xiaomi has an antivirus in Miui, based on the Avast engine, so you do have an antivirus.
Then, I recommend you to use a biometric recognition (face or fingerprint), because if someone has your unlock password, anyone can use your mobile.
Content filtering apps => Use the application "Dois-je Répondre" or "Orange Anti-spam" to block unwanted calls
Phone & Caller ID apps => Phone application for your mobile